Highly regioselective Ir-catalyzed β-borylation of porphyrins via C-H bond activation and construction of β-β-linked diporphyrin

Article abstract of DOI:10.1021/ja051073r

Highly regioselective borylation of 5,15-diaryl- and 5,10,15-triarylporphyrins has been achieved by the reaction with bis(pinacolato)diborane under iridium catalysis. Borylated porphyrins are a useful building block for constructing porphyrin arrays, hence offering an effective route to β-functionalized porphyrins. Copyright

Full text of DOI:10.1021/ja051073r

Published on Web 05/12/2005
Highly Regioselective Ir-Catalyzed â-Borylation of Porphyrins via C-H Bond
Activation and Construction of â-â-Linked Diporphyrin
Hiroshi Hata, Hiroshi Shinokubo,* and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, PRESTO & CREST, Japan Science and
Technology Agency (JST), Kyoto 606-8502, Japan
Received February 19, 2005; E-mail: osuka@kuchem.kyoto-u.ac.jp; hshino@kuchem.kyoto-u.ac.jp
Scheme 1
Directly linked multiporphyrinic molecules have received much
attention because of their unique properties.1 We have developed
an efficient route to meso-meso-linked porphyrins via Ag(I)-
mediated coupling of meso-unsubstituted porphyrins.1d,e The cross-
coupling reaction between bromo- and borylporphyrins is also a
powerful tool.2 Consequently, borylated porphyrins that can be
available via borylation of meso-bromoporphyrins are highly useful
building blocks.3
Recently, organoboron chemistry has been booming in fruitful
combination with transition metal catalysis. The Suzuki-Miyaura
reaction has become the most frequently used procedure for cross-
coupling reactions.4 Furthermore, iridium-catalyzed direct borylation
of aromatic compounds via C-H bond activation clearly demon-
strates the power of transition metal catalysis and has opened up a
new stage of organoboron chemistry.5
In connection with the synthesis of porphyrin arrays, we have
attempted the synthesis of borylated porphyrins via the iridium-
catalyzed direct borylation. We found that treatment of 5,15-bis-
(3,5-di-tert-butylphenyl)porphyrin (1a) with bis(pinacolato)diborane
Chart 1
in the presence of a catalytic amount of [Ir(cod)OMe]2 and 4,4′-
di-tert-butyl-2,2′-bipyridyl (tbbpy) in 1,4-dioxane provided monobo-
rylated porphyrin in 43% yield. Bisborylated porphyrins were also
obtained in 14% combined yield as a mixture of two regioisomers
(1:1). The reaction proceeded very cleanly without any byproducts
1
with recovery of 1a (37%). To our surprise, the H NMR spectra
of the borylporphyrin products 2a, 3a, and 4a indicated that
borylation took place at the â-position adjacent to the unsubstituted
meso-position. Typically, in the case of 2a, substantial downfield
shifts due to the introduced boryl group were observed for two
singlet signals, meso-H4 (δ ) 11.01) and â-H5 (δ ) 9.68), while
1
the H NMR spectrum of 1a exhibits meso-H1, â-H2, and â-H3 at
10.32, 9.41, and 9.13 ppm, respectively. On the basis of these
results, we concluded that â-H2 in 1a was selectively replaced with
the boryl group to provide 2a. This has been confirmed by the X-ray
diffraction analysis (vide infra). To our knowledge, this is the first
example of peripheral metalation of porphyrins via C-H bond
activation that proceeds with unprecedented regioselectivity of â-
over meso-position.
borylation of Ni- and Cu-porphyrins 1b and 1c can be also carried
out to provide the monoborylated porphyrins 2b and 2c in 47 and
44% yields, in a manner similar to the free base 1a. Unfortunately,
the reaction was sluggish with Zn-porphyrin 1d because of its low
solubility in dioxane. Borylation of 5,10,15-tris(3,5-di-tert-bu-
tylphenyl)porphyrin (5) provided the desired 2-borylporphyrin 6
in 57% yield.
Furthermore, exhaustive borylation of 5,10,15-triarylporphyrin
5 and 5,15-diarylporphyrin 1a with excess diborane for a prolonged
reaction period furnished 2,18-bis- and 2,8,12,18-tetraborylated
porphyrins 7 and 8 in 80 and 73% yields, respectively (Chart 1).
The structure of tetraborylated porphyrin 8 has been clearly
elucidated by the X-ray crystallographic analysis (Figure 1).9
Obviously, the sterically less hindered â-position was borylated with
high regioselectivity. Introduction of four boryl groups does not
result in any distortion of the flat porphyrin structure. Interestingly,
With regard to the regioselectivity, most of known electrophilic
reactions toward 5,15-disubstituted porphyrins, such as halogena-
tion, nitration, and Vilsmeier reactions, occur selectively at the
meso-position.6,7 Nucleophiles, such as alkyllithiums, also attack
exclusively at the meso-position of meso-unsubstituted porphyrins.8
It is worthy to note that the most reactive meso-position remained
untouched during this process. Consequently, the present direct
â-borylation offers a unique method for regioselective modification
of relatively simple porphyrins to functionalized ones. We assume
that the observed regioselectivity is mainly determined by steric
factors. Thus, tetrakis(3,5-di-tert-butylphenyl)porphyrin, which has
only congested â-pyrrolic protons, was recovered unchanged. The
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C O M M U N I C A T I O N S
between 2a and meso-bromoporphyrin 11. Importantly, these meso-
aryl â-linked porphyrin dimers, the â-â-linked one in par-
ticular, are quite difficult to synthesize due to limited availability
of proper â-functionalized porphyrin precursors. Diporphyrin
products 9 and 12 have unsubstituted meso-positions, which allow
further functionalization via conventional transformations, such as
regioselective dibromination with NBS from 9 to 10 in good yield.
The product is also useful for further fabrications of porphyrin
arrays.
In summary, we have achieved highly regioselective borylation
of meso-arylporphyrins via C-H bond activation under iridium
catalysis. This protocol offers an effective method for â-function-
alization of porphyrins, which is otherwise inaccessible or entails
multistep synthesis. One key feature is that the reaction does not
hurt the usually reactive meso-positions. Functionalization of
â-borylated porphyrins by taking advantage of rich organoboron
chemistry and exploitation of them for fabrication of multiporphy-
rinic compounds are currently underway in our laboratory.
Figure 1. X-ray structure of 2,8,12,18-tetraborylated porphyrin 8. Hydrogen
atoms on the aryl and dioxaborolanyl groups are omitted for clarity.
Scheme 2
Acknowledgment. This work was partly supported by a Grant-
in-Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan.
Supporting Information Available: General procedures, spectral
data for compounds, absorption and fluorescence spectra, and calculated
MOs by the DFT method. CIF file for the X-ray analysis of 8. This
material is available free of charge via the Internet at http://pubs.acs.org.
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reflections, 6490 unique reflections, R ) 0.0723, Rw ) 0.2142 (all data)
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the dioxaborolane rings are kept rather coplanar to the porphyrin
ring, which is favorable for the electronic interaction between the
vacant orbital of boron and the π-orbital of the porphyrin. In
contrast, the dioxaborolane plane is tilted at 52.0° to the por-
phyrin plane in the meso-borylporphyrin synthesized by Therien
et al. The B-C bond lengths (1.544 and 1.555 Å) were some-
what shortened as compared with that of meso-borylporphyrin (ca
1.57 Å). The Soret bands as well as the Q-bands of 2a and 8 are
red-shifted compared to that of 1a along with broadening of the
Soret band (Supporting Information). This is probably due to the
lowering of one of the degenerate LUMOs by the electron-
withdrawing boryl group, which is in line with the DFT calculations
at the B3LYP/6-31G* level (Supporting Information). The fluo-
rescence spectra of 2a and 8 are also red-shifted (Supporting
Information).
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2).11 In addition, construction of meso-â-linked diporphyrin 12
was accomplished effectively via the Suzuki-Miyaura coupling
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